In a dry electrophotography method, the toner particles used when converting (i.e., developing) an electrostatic latent image to form a visible image are normally formed by (i) premixing a thermoplastic binding resin (hereafter “binder resin”), a charge controlling agent, magnetic particles, and an external additive particle, (ii) melting and kneading the mixture, (iii) pulverizing the result, and (iv) classifying the resultant particles to form toner particles having a desired particle diameter. After this process, frictional electrification is performed to accumulate a predetermined amount of positive or negative charge on the surfaces of these toner particles, with the charged particles being used to develop an electrostatic latent image.
The electric charge that accumulates on the surfaces of the toner particles due to the frictional electrification performed here needs to be either positive or negative depending on the type of photoconductive photosensitive roll used to form an electrostatic latent image. Sufficient charge needs to be accumulated during the frictional electrification so that the electrostatic latent image can be properly developed to form a visible image. For these reasons, it is commonplace to mix and disperse a charge controlling agent and a conductive substance into binder resin so as to control the polarity of the charge and the amount of charge that accumulates on the surfaces of the toner particles, with inorganic fine powders, such as silica, aluminum oxide, titanium oxide and zinc oxide, usually being added for this purpose. However, as such inorganic fine powders are usually hydrophilic, there is the problem that the charging characteristics of the toner particles vary greatly with environmental conditions such as humidity.
The effects of environmental conditions such as those described above are conventionally countered by treating the surfaces of particles of these inorganic fine powders with a hydrophobic agent or by introducing a polar functional group.
As one example, JP 52-135739A discloses a technology that uses a metal oxide, which has been surface-treated with an amino-silane coupling agent, to introduce a polar functional group. JP 10-3177A discloses a toner where a titanium compound formed by a reaction between Ti(OH)2 with a silane coupling agent is used as an external additive particle. JP 5-181306A discloses an electrostatic latent image developer where fine particles of an abrasive agent, such as alumina or zirconia, are fixed to the surfaces of toner core particles and the ratio of the particle diameter of the toner core particles to the particle diameter of the fine abrasive agent particles is controlled. With this kind of electrostatic latent image developer, a superior abrasive effect is achieved for the surface of a photosensitive roll, so that a large cleaning mechanism such as a cleaning brush does not need to be used. As a result, image forming apparatuses can be made smaller, with there also being additional benefits regarding blurring phenomena, image density, and background printing (fogging).
However, as the amino-silane coupling agent is hydrophilic, the developed disclosed by JP 52-135739A suffers from a dramatic fall in fluidity and charging characteristics when used in high temperature and high humidity environment. As for the titanium compound disclosed as an external additive particle in JP 10-3177A, the average particle diameter of the titanium compound is extremely small, so that the compound is susceptible to coagulation, making it difficult to handle. As the abrasive effect is also poor, an extreme increase in charge occurs, thereby increasing the likelihood of problems such as decreases in image density, background printing, and blurring phenomena. With the electrostatic latent image developer disclosed in JP 5-181306A, while a desired abrasive effect can be achieved for the surface of the photosensitive roll, the charging characteristics are unstable, and the durability of the toner has not always been satisfactory.
JP 62-113158A, JP 64-62667A, and JP 5-188633A disclose toners to which hydrophobic silica and (anatase-type) titanium oxide have been added. However, with such toners, friction results in the (anatase-type) titanium oxide becoming embedded in the toner particles, which results in the problem of the charging characteristics becoming unstable.
JP 2000-128534A discloses a toner to which hydrophobic titanium oxide is added. This hydrophobic titanium oxide is formed by treating the surfaces of (i) hydrous titanium oxide, and/or (ii) rutile-type titanium oxide that includes some anatase-type titanium oxide, with a silane coupling agent. The hydrophobic titanium oxide is prevented from becoming embedded inside the toner particles by setting the major axis diameter of the hydrophobic titanium oxide in a range of 0.02 to 0.1 μm and the axial ratio in a range of 2 to 8. However, such hydrophobic titanium oxide is difficult to manufacture, has a low bulk density and is difficult to form with stable charging characteristics.
To the contrary in recent years, recognition marks called “fonts” have been used on checks, securities, bills, tickets, etc, to prevent forgery and tampering. Forgery prevention technologies that use fonts are normally referred to as “MICR (Magnetic Ink Character Recognition) systems”, with examples of such being disclosed by JP 2-134648A, JP 5-80582A, and U.S. Pat. No. 5,034,298. In more detail, recognition marks that are made up of such fonts are composed of combinations of numbers and symbols, and are printed on the surfaces of checks and the like to prevent forgery. These recognition marks composed of fonts are formed using a magnetic ink in which a predetermined amount of magnetic particles has been dispersed in a binder. As a result, by using the magnetism of the magnetic particles, it is possible to judge whether the checks or the like are genuine or fake from information outputted by a specialized reader that reads the fonts in the recognition marks. These recognition marks composed of fonts are visible to the human eye, so that an initial judgement as to whether the stamps, etc. are genuine can be made by simply looking at them. As a result, unlike barcodes, for example, there is the advantage that a simple and fast judgement can be made before the specialized reader is used. As examples, a screen printing method or a gravure printing method can be used to print fonts using the magnetic ink, though in recent years more attention is being paid to using printers as an easy and fast way of printing fonts. When a printer is used to form an image with a magnetic ink, the magnetic ink used is usually referred to as a “MICR toner” or a “MICR printer magnetic toner”. MICR toners are usually composed of (1) MICR toner particles that are made up of (i) the binder resin composed of a thermoplastic resin, (ii) a wax or a wax derivative as a surface lubricant, (iii) magnetic particles, (iv) an inorganic powder, etc., and (2) an external additive particle. In more detail, the above materials are evenly kneaded, and then pulverized and classified to form MICR toner particles. A process to add external additive particles, such as silica and an abrasive agent, is then performed to finally form one type of toner whose average particle diameter is in a range of around 4 to 15 μm. However with a conventional MICR toner, the residual magnetism has to be sufficiently high for reading to be performed successfully, so that the charge distribution of the remaining toner in a developing apparatus becomes broader as the printing operations are repeatedly and long performed. This results in problems such as a decrease in image density, increased probability of background printing, and a high incidence of read errors for the recognition marks are formed.
For this reason, JP 4-358164A, JP 4-358165A, and JP 7-77829A disclose MICR toners that include a binder resin (polyolefin resin) and magnetic particles and have two types of magnetic particles mixed with and dispersed in the binder resin. In more detail, the presence of two types of magnetic particles in these MICR toners results in the residual magnetism being kept within a range of 4.0 to 7.0 emu/g. However, it is not possible to raise the residual magnetism of a MICR toner by simply combining two types of magnetic particles, and if the residual magnetism of the MICR toner is kept within a range of 4.0 to 7.0 emu/g, problems such as a high incidence of read errors are still observed.
For this reason, by performing a through investigation of the problems with the conventional technology, the inventors of the present invention found that it is possible to overcome the problems that were observed for conventional toners by                1. causing interacting effects by adding anatase-type titanium oxide to improve abrasion and adding rutile-type titanium oxide to sharpen the charge distribution, or        2. adding an amount of rutile-type titanium oxide so that the specific volume of rutile-type titanium oxide falls in a specific range.        
These techniques constitute the present invention.
In other words, it is an object of the present invention to provide (i) a toner which exhibits stable charging characteristics with an even charge distribution, no decrease in frictional electrification or increase in charging ability with time/use, and has excellent fluidity, environmental independence, and durability, and (ii) an image forming method that uses the toner.